Shape charges are specialized explosives molded into special shapes that control the direction of their explosion. The directional force of these explosions results in incredible precision, allowing demolition crews to drop enormous skyscrapers into their own basements.
The most frightening and fascinating use of shape charges is as the fuse for nuclear warheads. To explain how they fulfill this role, it is first necessary to understand how nuclear fission works. Neither of us are nuclear physicists, so I’m going to give the basic version of this.
An atom is made up of a nucleus, orbited by a number of electrons. Electrons are negatively charged particles, which are attracted to the nucleus by the positive charge of the protons that make it up. Remember, opposites attract. But the nucleus is not made solely of protons. A good portion of its weight (or mass, as scientists would prefer to call it), is made up of neutrons. Neutrons, as their name suggests, are neutral – they have no charge.
For the purposes of a nuclear reaction, the nucleus of the atom can be considered a target. If you try and hit it with an electron or a proton, you will always miss because these are charged particles and will veer one way or the other when they approach the charged components of the atom. When scientists discovered the neutron, however, they had found their magic bullet.
Since the neutron has no charge, it does not interact with the electrons orbiting the nucleus or the protons in the nucleus itself as it closes on the target. When the neutron strikes the nucleus, it splits it into pieces and releases a few of the neutrons formerly trapped inside.
These extra neutrons that split off from the nucleus as it breaks up can be used to set up a chain reaction. If the conditions are right, they will in turn strike the nuclei of other atoms, causing them to split or “fission” as well, in turn causing other atoms to fission and so on. Each time fission takes place, part of the mass of the nucleus involved is actually converted into energy. This is described by Einstein’s famous equation, E=mc2, which says that matter and energy are equivalent.
Because a chain reaction keeps going, energy is continually released. When the rate of energy-release is controlled, what you have is a nuclear power plant. What you have when it is uncontrolled is a nuclear weapon or a Chernobyl.
The trick to keeping a chain reaction going in the first place is to make sure that each neutron released when one atom fissions actually goes on to hit another atom. To make this happen you need a lot of atoms lying around. There are a limited number of substances that will fission properly to create a chain reaction.
One of these is U-235, an isotope of uranium. The amount of uranium necessary to ensure a chain reaction is known as the critical mass. The atomic bomb exploded over Hiroshima was a uranium bomb, also called a gun-type bomb. In this weapon, a large piece of uranium is placed at one end of what is effectively a rifle-barrel. A bullet, also made of uranium, is fired through the barrel into the larger piece. The total mass of the two exceeds the critical mass and the bomb goes off as a result.
U-235 is exceedingly rare and hard to refine from its more common counterpart U-238. A more efficient type of bomb quickly replaced it: the plutonium bomb. Plutonium is an element generally not found in nature. It is produced and harvested from nuclear reactors. In one case, a natural deposit of uranium in Africa acted as a natural reactor for some time; but if it produced plutonium, the stuff has long since broken down into more stable elements.
To make a plutonium bomb, scientists first had to produce plutonium. The element, as it turns out, is highly unstable, which makes critical mass a touchy subject. Nominally safe amounts of plutonium can hair-trigger and cause an explosion. So rather than try and construct a gun-type bomb of plutonium, scientists used a very small amount of plutonium – about the size of a grapefruit – and crushed it down until its atoms were so tightly packed that there were enough of them in one place to begin a chain reaction. Until it is compressed, the tiny amount of plutonium is idle. This made the bomb safe on the shelf and still just as deadly (even more so) than the uranium bomb.
Compressing the grapefruit-sized ball of plutonium to the size of a walnut takes an enormous amount of force applied equally on all sides at precisely the same moment. This is why shape charges are used as the trigger. The plutonium bomb itself releases so much energy that it can be used to fuse hydrogen into helium, which releases even more energy. This is where the hydrogen bomb comes from.
All this horrifying gadgetry was monitored by the Atomic Energy Commission, which later became the Department of Energy. While it still regulates nuclear labs across the country, the D.O.E. now has another pet project – the Human Genome Project.
Josh Braun is the Daily Nexus science editor.